feat(core): add git commit indexing (#92)

[prosdev]

Summary

Implements the GitIndexer for indexing git commits into the vector store, enabling semantic search over commit history.

Changes

GitIndexer (packages/core/src/git/indexer.ts)

Core Functionality:

• index(options) - Extract and index commits into vector store

  • Configurable commit limit (default: 1000)
  • Date range filtering (since/until)
  • Author filtering
  • Merge commit exclusion
  • Progress reporting callback
  • Batch processing for efficiency

• search(query, options) - Semantic search over commit messages

  • Returns full GitCommit objects
  • Filters by commit document type
  • Score threshold support

• getFileHistory(path, options) - Get commits for a specific file

  • Uses git --follow for rename tracking

Document Structure:
Each commit is stored with:

• Text: Subject + body + file paths (for better semantic matching)
• Metadata: hash, author, date, stats, issue/PR refs, full commit object

Factory Function

• createGitIndexer(repoPath, vectorStorage) - Convenience factory

Tests

• 17 comprehensive unit tests covering:
  • Commit extraction and indexing
  • Filtering options (limit, date, author)
  • Error handling (extraction, storage)
  • Progress reporting
  • Batch processing
  • Search functionality
  • File history retrieval
  • Document structure validation

Design Decisions

1. Rich Text for Embedding: Include file paths in embedding text so searches like "changes to auth" find commits that touched auth files
2. Full Commit in Metadata: Store complete GitCommit object in _commit field for retrieval without re-parsing
3. Type Filtering: Use type: 'commit' metadata to distinguish from code documents
4. Batched Storage: Process in configurable batches to handle large histories

Closes

Closes #92

Part of

Epic: Intelligent Git History (v0.4.0) #90